Correlation between the mutation rates for ampC derepression and species or ampC genotypes in Enterobacter cloacae complex

Background In this study, we investigated the correlation between the mutation rates for ampC derepression and species or ampC genotypes in Enterobacter cloacae complex (ECC) susceptible to ceftriaxone. Non-duplicate ceftriaxone-sensitive ECC isolates (90) were obtained from September 2021 to January 2023 at Ruijin Hospital, Shanghai. hsp60 genotyping and PCR were used for species and ampC identification, respectively. Thirteen strains with the negative ampC amplification results were sequenced, and the mutation rates for ampC derepression were determined by performing Luria–Delbrück fluctuation analyses. Results Among these isolates, E. hormaechei was the most prevalent (54.44%), followed by E. roggenkampii (12.22%), E. cloacae (11.11%), E. bugandensis (10.00%), E. asburiae (4.44%), and E. kobei (4.44%). For E. ludwigii , E. mori , and E. sichuanensis , only a single strain was identified. There were 72 strains with bla ACT , 13 with bla MIR , and five with bla CMH . The ampC -derepressed mutation rate was (2.25 ± 1.81) × 10 −8 for E. asburiae , (3.21 ± 2.96) × 10 −8 for E. bugandensis , (6.06 ± 11.95) × 10 −8 for E. cloacae , (1.12 ± 3.44) × 10 −7 for E. hormaechei , (7.76 ± 11.41) × 10 −8 for E. kobei , (3.99 ± 9.65) × 10 −8 for E. roggenkampii , 5.87 × 10 −8 for E. ludwigii , 1.12 × 10 −7 for E. mori , and 2.17 × 10 −7 for E. sichuanensis. The mutation rate was (8.94 ± 28.61) × 10 −8 for bla ACT , (2.62 ± 2.16) × 10 −8 for bla CMH , and (8.10 ± 13.84) × 10 −8 for bla MIR . Conclusions ECC was found to have high mutation rate−inducible AmpC production with no species or ampC genotype differences. This highlights an important clinical concern, i.e., the high risk of treatment failure with third-generation cephalosporins in individuals with inducible AmpC-containing ECC.